1. Field of the Invention
The present disclosure relates to an array substrate. More particularly, the disclosure relates to an array substrate in which deterioration of an oxide semiconductor layer of a thin film transistor is prevented by blocking inflow of hydrogen into the oxide semiconductor layer, and a method of fabricating the same.
2. Discussion of the Related Art
As an information-based society progresses, display devices processing and displaying a large amount of information has rapidly advanced and various flat panel displays (FPDs) have been developed. Specifically, FPDs such as a liquid crystal display (LCD) device, a plasma display panel (PDP) device, an organic light emitting diode (OLED) display device and a field emission display (FED) device having a superior performance with a thin profile, a light weight, and a low power consumption have replaced the cathode ray tube (CRT).
Among various FPDs, the LCD device having a high contrast ratio, an advantage in displaying a moving image and a low power consumption has been used in various fields such as a notebook, a monitor and a television. The LCD device uses an optical anisotropy and a polarization property of a liquid crystal molecule. Since the liquid crystal molecule has a long and thin profile, the liquid crystal molecule has an optical anisotropy of an orientation in arrangement and a polarization such that an arrangement direction of the liquid crystal molecule is changed according to an intensity of an electric field.
In addition, the OLED display device has superior properties such as a high brightness and an ability to be driven by a low voltage. Since the OLED display device has an emissive type, the OLED display device has a high contrast ratio and a thin profile. The OLED display device has an advantage in displaying a moving image due to a short response time of several micro seconds (μsec). The OLED display device has no limitation on a viewing angle and is stable even in a low temperature. Since the OLED display device is driven by a low voltage of 5V to 15V in direct current (DC), it is easy to fabricate and design a driving circuit.
The LCD device and the OLED display device include an array substrate having a thin film transistor (TFT) for turning on and off a pixel region. A gate line and a data line crossing each other to define the pixel region are formed on the array substrate. In addition, at least one TFT is formed in the pixel region. The TFT functions as a switching element for turning on and off the pixel region and as a driving element for driving the pixel region to display an image. Specifically, at least two TFTs are formed in the pixel region of the array substrate for an OLED display device.
The TFT in the pixel region has various structures according to a material for a semiconductor layer. The semiconductor layer may include one of amorphous silicon, an oxide semiconductor material and polycrystalline silicon, and the array substrate includes the TFT having a top gate structure or a bottom gate structure according to the material for the semiconductor layer.
Among the TFTs having the semiconductor layer of various materials, the TFT having an oxide semiconductor layer of an oxide semiconductor material has been the subject of recent research.
The TFT having the oxide semiconductor layer has superior conductivity of carriers to the TFT having the semiconductor layer of amorphous silicon. In addition, the TFT having the oxide semiconductor layer has a simplified fabrication process as compared with the TFT having a polycrystalline silicon semiconductor layer requiring a doping with impurities.
However, the array substrate having the oxide TFT includes a plurality of insulating layers of an organic material or an inorganic material such as a gate insulating layer, an etch stopper and a passivation layer over or under the oxide semiconductor layer. In addition, the array substrate for an OLED display device further includes a planarization layer and a bank layer.
The plurality of insulating layers, specifically the insulating layers of silicon nitride, have a large amount of hydrogen and the hydrogen of the plurality of insulating layers is diffused into the oxide semiconductor layer. As a result, the oxide semiconductor layer is deteriorated by the hydrogen and a lifetime of the oxide semiconductor layer is reduced. In addition, since the hydrogen changes the oxide semiconductor layer into a conductor, the oxide semiconductor layer does not function as a semiconductor layer. As a result, a property of the TFT having the oxide semiconductor layer is deteriorated.
Further, the current and voltage property of the TFT is shifted along a negative direction (negative threshold voltage shift). When the current and voltage property of the TFT is shifted along a negative direction, light emitting diodes using the TFT as a driving element have different emission brightness properties according to a position in the OLED display device and the display quality of the OLED display device is deteriorated due to the non-uniformity in brightness.